A. Technical Field
The present invention relates generally to the management of skew within a polarized multi-channel optical system, and more particularly, to frame alignment and skew compensation across polarized optical channels within an optical transport system.
B. Background of the Invention
The capacity of transport systems is continually increasing to provide larger amounts of available bandwidth to clients. These transport systems are able to communicate large amounts of data using optical networking technologies. In certain systems, wavelength division multiplexed (hereinafter, “WDM”) transport systems communicate this data on multiple wavelengths between terminal nodes. These WDM systems offer a network provider scalable bandwidth without having to significantly expand the physical infrastructure of a network to realize this additional bandwidth.
Certain transport systems are designed to receive a client signal, re-format the client signal and transmit this reformatted client signal over a long-haul connection. The client signal is subsequently reconstructed at a receiver in the transport system and delivered to the client network. The transport system may operate in accordance with various standard protocols, such as the Optical Transport Network (hereinafter, “OTN”) protocols, or proprietary formats and procedures.
Transport systems are designed to interface with various types of client networks. In so doing, the transport system maps data from a client signal into a transport frame in which the data propagates across a transport connection. This mapping procedure is typically specific to the protocol of the client signal and the format of the transport system so that the client signal may be completely reconstructed at a transport receiver. The transport terminal nodes, both transmitter and receiver nodes, are generally able to operate in different modes depending on the type of client signal that is being processed. For example, a transport terminal node may map a client SONET frame into a transport frame using a first mapping procedure or an Ethernet frame into a transport frame using a second mapping procedure.
The transmission characteristics of the client data across the transport connection may depend on both the protocol and rate of the particular client signal. If a client signal is transmitted at a higher rate than the channel rate of the transport system, then the client data is transmitted across multiple channels in the transport system. This multi-channel transmission of client data preserves the rate of the client signal and allows efficient reconstruction of the client signal at a transport receiver node.
Transport technologies have been and are currently being developed to enable efficient communication of this client data on a transport system. In certain transport systems, the client data is transmitted across the transport system on multiple wavelengths. In certain other transport systems, the client data is transmitted across the transport system on multiple polarized channels, some of which may be on the same optical wavelength. Certain other transport systems may use a combination of wavelengths and polarized channels to communicate the client data across the transport system. Each of these wavelengths and/or polarized channels is received at a transport receiver node and the client signal is reconstructed by rebuilding and serializing the client data.
The transport receiver node requires that the differential latency between the channels, including the polarized channels, fall below a certain threshold in order for proper reconstruction of the client signal to occur. This differential latency or skew describes timing mismatches between the channels at the transport receiver node.
FIG. 1 generally illustrates an exemplary polarized multi-channel transport system. The transport system 100 comprises a first terminal node 110 that receives a high-rate client signal from a first client node or network gateway 120. The first terminal node 110 divides the client signal across multiple polarized optical channels and transmits the optical channels to a second terminal node 130. The second terminal node 130 reconstructs the high-rate client signal from the polarized optical channels and transmits the reconstructed signal to a second client node 140. One skilled in the art will recognize that the polarized multi-channel transport system 100 may be used to transport various types of client signals and signal rates.
An example of such a client signal is a 100 gigabit Ethernet signal that is framed and transmitted across the polarized multi-channel transport system 100. The 100 gigabit Ethernet signal may be transported on two polarized 50 gigabit channels, both of which are on the same optical wavelength or on different optical wavelengths. As previously mentioned, the transportation of these polarized and non-polarized optical channels requires that timing between these channels be maintained within a threshold. If this timing becomes too large (i.e., the signal skew is too large), then reconstruction of the 100 gigabit Ethernet signal becomes more difficult and errors may be generated in this process.
Skew between channels, both multi-wavelength and polarized channels, should be managed to ensure proper operation of the transport system 100. Skew between polarized channels may result from a number of factors relating to both the transmission medium and transport nodes. Polarization modal dispersion (hereinafter, “PMD”) may cause differential latency between the channels during transport because of differing propagation velocities of the channels. These differing propagation channel velocities are caused by the interactions of the polarized channels with the fiber. For example, a first channel propagating in a first polarization mode within a piece of fiber may have a slightly different velocity than a second channel propagating in a second polarization mode within the same piece of fiber. This difference in channel velocities results in a skewing of the timing between the channels.
Skew between polarized channels may also result in differences in signal path lengths between the channels. For example, polarized channels may be separated and communicated on different signal paths within a node. One cause of these slight differences in lengths is variations of fiber pigtails lengths that propagate the polarized channels. These different fiber pigtail lengths may also result in skewing between channels. Other components, both optical and electrical, within a transport node may also introduce skew between these polarized channels.
In order for the transport system to function properly, this skew between polarized channels needs to be managed. As the client signal rate increases, the management of the skew may become more complex because of the relatively larger amount of data that is propagated through the transport network. In effect, the transport network must provide the client more bandwidth, which means that either the number of channels serving the client is increased or the channel data rates are increased. In either scenario, maintaining the timing relationship of this client data as it propagates through the transport network becomes more complex.